Some diseases are very predictable in terms of age of onset and severity of symptoms. Such a disease is typically “expressed” in the same way in each affected individual. But some conditions, for example Toller PRA, don’t fit this description. They might have very different ages of onset, different degrees of severity, and/or different rates of progression even within the same line, the same pedigree, or even the same litter. One confusing result of reduced or variable expressivity is that a dog can be affected according to a DNA test, yet show no clinical signs of disease until much later, or show only mild and slowly progressing clinical signs of the disease. This dog must not be confused with a case of false positive.